Present day portable generators typically make use of a synchronous alternator or cycloconverter for providing the desired power output, which is typically 120 VAC or 240 VAC. Important considerations for any portable generator are:
Voltage regulation;
Dual voltage output capability;
Idle voltage and frequency;
Frequency tolerance;
Harmonic distortion:                Induction motor operation        Charger operation        
Grounding configuration;
4-blade (120–240 volt) twist-lock compatibility;
Response to load changes; and
Size and weight.
With regard to idle voltage and frequency, it is far easier to provide 120 volts and 60 Hz at idle using electronic solutions (i.e., inverter technology) than it is with synchronous alternators. However, sufficient voltage “head room” is still required. This higher voltage requires more turns in the alternator coils resulting in an increased coil resistance and reduced system efficiency.
Harmonic distortion present in the output waveform of a portable generator is another important consideration that must be addressed. While waveform purity is of little importance to constant speed universal motor-powered portable power tools, it is an important consideration when running induction motors and chargers. Induction motors will run on distorted waveforms, but the harmonic content of the input will be converted to heat, not torque. The extra heating from the harmonics must be quantified if a inverter topology which produces a distorted waveform is to be implemented. A sine wave pulse width modulated (PWM) inverter will produce excellent waveforms with only some high frequency noise, but they are likely to require full H-bridges which, traditionally, have not been easily adaptable to the North American grounding convention and the 4-blade twist-lock wiring convention.
With regard to grounding configurations, in North America, the standard grounding convention requires that one side (neutral) of each 120 volt circuit is grounded. This means that 240 volt circuits have floating grounds.
Still another important consideration is 4-blade (120–240 volt) twist lock compatibility. This convention requires four wires: ground, neutral, 120 volt line 1 and 120 volt line 2. Each 120 volt circuit is connected between a 120 volt line and neutral. The 240 volt circuit is connected between the 120 volt line 1 and the 120 volt line 2.
The ability of a generator to respond to load changes is still another important consideration. All inverter topologies will provide a faster response to load changes than a synchronous alternator, due to the large field inductance used by a synchronous alternator.
Concerning size and weight, it would also be desirable to make use of inverter topology because virtually any inverter topology will provide size and weight benefits over that of a synchronous alternator. However, trying to produce sine waves from a two half bridge circuit may require large capacitors that would reduce the benefit of volume reduction provided by the inverter topology.
Cycloconverters have been used in generator systems to convert the AC voltage generated by the generator to the desired AC output voltage. Electrical systems using cycloconverters typically have an AC voltage source to the cycloconverters that is fairly stiff (low source impedance). Consequently, the AC phasing information for commutation of the SCRs of the cycloconverters can be directly derived from the 3-phase AC voltages provided to the cycloconverters. Suitable filtering is necessary to remove the commutation notches introduced by SCR switching/commutation. However, permanent magnet generators provide a very soft AC source in that they have significant series reactance. This presents two problems for control of the SCRs of the cycloconverter in a generator systems using a permanent magnet generator. First, the AC voltage waveforms are significantly disturbed by the switching of the SCRs of the cycloconverter and thus would require significant filtering. Second, the reactance of the permanent magnet generator introduces a significant phase shift between the back-emf voltage waveforms of the permanent magnet generator (which cannot be measured) and the AC voltages at the outputs of the permanent magnet generator (terminal voltages), especially as the generator system is loaded. This load dependent phase shift can't be eliminated by a simple filter.
Generators having two isolated 120 VAC outputs that can be switched between 120 VAC parallel connection mode (120 VAC mode) to a 240 VAC series connection mode (240/120 VAC mode) would typically use a multi-pole switch, as shown in FIG. 10. With reference to FIG. 10, generator system 1000 is shown as having two isolated 120 VAC sources 1002, 1004, which could be cycloconverters such as cycloconverters 42, 44 described below. Generator system 1000 also has a 120 VAC output, shown illustratively as resistance 1006, a 240 VAC output, shown illustratively as resistance 1008, and a switch 1010 that switches generator system 1000 between the 120 VAC parallel connected mode where sources 1002 and 1004 are connected in parallel and the 240 VAC series connected mode where sources 1002 and 1004 are connected in series.
Positive output 1014 of 120 VAC source 1004 is connected to ground and to one side of 120 VAC output 1006. Negative output 1018 of 120 VAC source 1004 is coupled to the other side of 120 VAC output 1006 and to one side of 240 VAC output 1008. Switch 1010 switches positive output 1012 of 120 VAC source 1002 and negative output 1016 of 120 VAC source 1002 to switch 120 VAC sources 1002, 1004 between the 120 VAC parallel connected mode and the 240/120 VAC series connected mode as described below.
Switch 1010 is a multi-pole switch, such as a double pole relay, as shown in FIG. 10. When in the parallel connected 120 VAC mode, positive output 1012 of 120 VAC source 1002 is connected to positive output 1014 of 120 VAC source 1004, and thus to ground, by switch 1010 and negative outputs 1016, 1018 of sources 1002, 1004, respectively are connected together by switch 1010. 120 VAC is provided at 120 VAC output 1006 by the parallel connected 120 VAC sources 1002, 1004.
In the 240 VAC series connected mode, positive output 1012 of 120 VAC source 1002 is connected through switch 1010 to the other side of 240 VAC output 1008, with the first side of 240 VAC output 1008 connected to the negative output 1018 of 120 VAC source 1004 as described above. The negative output 1016 of 120 VAC source 1002 is connected through switch 1010 to ground. 120 VAC is provided at 120 VAC output 1006 by 120 VAC source 1004 and 240 VAC is provided at 240 VAC output 1008 by the series connected 120 VAC sources 1002, 1004.